1. Field of the Invention
This invention relates to telemetering information from a remote location to a control station. More particularly, the invention relates to the monitoring at a local test station the position of switches located at the wellhead of a subsea petroleum well.
2. Description of the Prior Art
In the control and monitoring of some types of processes or instrumentation, the position of several remote switches or contact closures must be monitored. For example, to monitor the production of one type of subsea petroleum well, the position of four valves on the subsea wellhead structure must be monitored at an accessible location such as on the sea surface. Usually there are two limit switches associated with each valve, one to indicate the fully opened position and another for the fully closed position. Monitoring the position of these eight limit switches normally would require nine conductors including one common conductor and one conductor for each switch. Cable connectors are then required to have the same number of contacts.
However, due to the subsea environment of high pressure and salt water and the requirement to install the connectors using divers, submersible vehicles or other means, it is desirable to minimize the number of conductors and connector contacts for reliability and installation reasons. In addition, due to connector or cable corrosion, the value of the circuit impedance can change with time or from one installation to the next. Thus, it is desirable to utilize only a single pair of electrical conductors between the subsea location and the surface. Furthermore, the use of alternating current electrical signals for monitoring is highly desirable to minimize electrolysis and subsequent corrosion of the conductors or connectors. Also, for long term reliability reasons, it is not desirable to use a source of power at the wellhead structure. It is likewise desirable to minimize circuit complexity for reliability reasons.
One prior art system for monitoring the status of control valves and switches utilizes passive resonant circuitry such as that disclosed in U.S. Pat. No. 3,550,090 issued to Baker, Jr. et al., U.S. Pat. No. 4,027,286 issued to Marosko and U.S. Pat. No. 4,268,822 issued to Olsen. In these systems, frequency selective resonant circuits are placed near the individual switches. The resonant circuits are connected or disconnected or the resonant frequencies are changed by the closure of the switches. Decoding circuitry responsive to the change in resonance of the switch circuits is used to indicate the status of the switches. The coding and decoding circuitry is very complex. In addition, in the U.S. Pat. No. 4,027,286 system, the bandwidth of the conductive coupler limits the channel capacity so that the status of only six switches can be determined.
Another prior art telemetry system is disclosed in U.S. Pat. No. 4,136,327 issued to Flanders et al., wherein a two conductor line serves a dual purpose of supplying power into the borehole and communicating sensor data back up to surface. In the Flanders et al. system, phase modulation is used to transmit a binary encoded signal to the surface that is received by a digital computer. In this type of system, the remote system electronics is complex, which may degrade reliability in a subsea environment.
U.S. Pat. No. 4,459,760 issued to Watson et al., discloses a technique using serial encoding circuitry to convert digital signals into a frequency shift keyed modulated signal for transmission on a two conductor wire line. The circuitry required for this technique is quite extensive, requiring enclosures that are relatively inexcessible at the wellhead.
U.S. Pat. No. 4,103,337 issued to Whiteside, discloses a data transmission system wherein a digital processor generates a data request signal across a two wire transmission line to a remote sensor. Sensor interface circuitry includes an analog to pulse width converter and a decoder which generates a pulse width signal in response to the data signal indicative of the value of the analog signal generated by the sensor. A pulse width to digital converter converts the data signal to digital form which is terminated in response to the pulse width signal from the sensor interface. The signal stored in the pulse width to digital converter is transmitted to the digital processor to indicate the value of the parameter being sensed.